1. Field
Exemplary embodiments of the present invention relate to a non-volatile memory device, and a method for performing a program operation and an erase operation of the non-volatile memory device.
2. Description of the Related Art
Semiconductor memory devices may be divided into volatile memory devices and non-volatile memory devices based on whether data is to be stored even without a power supply. Non-volatile memory devices such as flash memory devices have both of the advantage of a Random Access Memory (RAM), which is capable of freely programming and erasing data, and the advantage of a Read Only Memory (ROM), which may retain data even without a power supply. In particular, NAND flash memory devices are widely used in a variety of applications for high-capacity data storage, because it is easy to increase the integration degree of the NAND flash memory device.
FIG. 1 illustrates a memory cell array of a NAND flash memory device.
Referring to FIG. 1, the cell array of a NAND flash memory device may include a plurality of cell strings, e.g., cell strings 100 and 110.
The cell strings 100 and 110 include, respectively, between drain selection transistors 101 and 111 and source selection to transistors 103 and 113, a plurality of memory cells coupled in series. The drain selection transistors 101 and 111 and the source selection transistors 103 and 113 are coupled with a drain selection line DSL and a source selection line SSL, and the plurality of memory cells are coupled with a plurality of word lines WL0 to WLN. The cell strings 100 and 110 are selectively coupled with bit lines BL through the drain selection transistors 101 and 111, and the cell strings 100 and 110 are selectively coupled with a common source line CSL coupled with a ground voltage end through the source selection transistors 103 and 113. The multiple cell strings 100 and 110 that are coupled with the bit lines BL, respectively, are coupled in parallel with the common source line CSL so as to form one memory cell block.
To record a data in a memory cell of the non-volatile memory device, that is, to program a memory cell of the non-volatile memory device, the data of all memory cells of a block corresponding to a program operation are to be erased prior to the program operation. The conventional block-based erase operation may be performed by floating a drain selection line DSL and a source selection line SSL, applying an erase voltage, e.g., 0V, to all word lines, and applying a high voltage, e.g., 20V, to a semiconductor substrate. Through the block-based erase operation, the threshold voltages of all the memory cells of the corresponding block may drop to 0V or lower to turn the memory cells into an erase state.
A non-volatile memory device has been developed to increase its integration degree within a smaller size. For example, the number of word lines implemented in one block is increased from 32 to 64. In short, the size of cell strings, i.e., the length corresponding to the number of word lines, of a memory cell array continues to increase, and this may cause degradation in the performance of a non-volatile memory device such as program disturbance.
FIGS. 2A and 2B illustrate a change in the threshold voltage distribution of memory cells due to program disturbance in a non-volatile memory device. Here, FIGS. 2A and 2B show a multi-level cell MLC capable of storing two bits of data as an example.
One memory cell MLC may have four data storage states. When the memory cells have a uniform threshold voltage distribution ideally as shown in FIG. 2A, the threshold voltage of a memory cell in an erase state is approximately 0V or lower and the first read voltage VR1 is approximately 0V.
However, in the actual memory cells, the threshold voltage distribution levels may all increase due to program disturbance and, that is, the distribution graph may move to the right along the threshold voltage level axis as shown in FIG. 2B. Particularly, this phenomenon may be significant in the distribution graph of memory cells in an erase state. Conventional technology has coped with the phenomenon by increasing the read voltage VR1 higher than 0V, but it may decrease a margin for the overall threshold voltage distribution of memory cells.
Also, as the size of cell strings of a memory cell array increases and thus the number of word lines in one block increases, the program disturbance may be more pronounced for the high-level word lines. FIG. 3 shows a change of the threshold voltage distribution of erase-state memory cells when a program operation is performed repeatedly in one block.
Referring to FIG. 3, when there are 64 word lines WL0 to WL63 in one block, the threshold voltage distribution level of the memory cells coupled with the high-level word lines (e.g., a word line WL63) may increase as the program operations are performed onto the block. This is because a program operation is generally performed in the sequence from low-level word lines (e.g., a word line WL0) to high-level word lines.
The program disturbance becomes more pronounced for the high-level word lines first because a pass voltage is also applied to the high-level word lines when the program operation is performed on low-level word lines, and second because as the memory cells coupled to the low-level word lines are programmed, the amount of current flowing the memory cells are decreased and thus the turn-on resistance of the memory cells coupled with the high-level word lines becomes great to raise the threshold voltage of the memory cells.